The oxychlorination of ethylene to DCE is carried out, as it is known, either in fluid bed or in fixed bed. In the first case, more uniform distribution of the temperatures in the reactor is obtained, in the other case, the management of reaction parameters is easier but, due to the low exchange coefficient among the catalyst granules and between the granules and the reaction gas, localized hot spot temperatures can occur having detrimental effects on the selectivity and useful life of the catalyst.
Hollow cylindrical granules are normally used which, thanks to S/V ratio (geometric surface to volume ratio) higher than that of the spheres and solid cylinders allow to obtain more efficient heat exchange and lower pressure drop through the catalytic bed, and consequently better temperature control along the bed and increased productivity of industrial reactors.
In spite of the above advantages, a hollow cylindrical granule has to be designed carefully since, otherwise several disadvantages become evident.
For example, if the ratio of the external to internal diameter (De/Di) of the hollow cylinder is greater than a certain value, the granules become too fragile and the bulk density of the catalyst decreases resulting in a decreased conversion per unit volume of the catalytic bed due to the lower presence of total content of the active catalyst phase.
A too high increase of De or the length of the cylinders maintaining constant the De/Di ratio can cause an inhomogeneous loading of the catalyst inside the tubes of the reactor and possible breakage of the granules with consequent increase of the pressure drop.
A catalyst in form of cylinders having De from 4 to 7 mm, Di from 2.0 to 2.8 nm, height from 6.1 to 6.9 mm is described in EP 1 053 789 A1.
This catalyst is reported to be advantageous with respect to the catalysts in form of hollow cylinders having length shorter than the external diameter described in U.S. Pat. No. 4,740,644 and the catalyst cylinders having longer length than the external diameter described in U.S. Pat. No. 5,166,120.
The catalysts of the latter cited US patent are also characterized by a total pore volume of at least 0.6 to 1.0 ml/g, wherein no pores smaller than 4 nm are present and at least 80% of the total pore volume is formed of mesopores with a diameter of 8 to 20 nm, the remainder being pores with diameter of more than 20 nm and up to 200 nm.
The catalysts of U.S. Pat. No. 5,166,120, according to the consideration made in EP 1 053 789, has the disadvantage of a too high bed void fraction which implies a lower amount of catalytic material present in the bed and consequently a lower specific productivity to DCE (g DCE/g catalyst.h) combined with high pressure drop due to breakage of the catalyst granules during the loading step. The activity of the catalysts is higher than that of catalysts the pore volume of which is mainly formed of pores with diameter of less than 8 nm.
Commercial hollow cylindrical catalysts are known having total pore volume of at most 0.40 ml/g wherein the micro and mesopores are the major component. The productivity of these catalysts is rather low.
Objects
It is an object of the present invention to provide catalysts for the fixed bed oxychlorination of ethylene to DCE in form of hollow cylindrical granules having rather high total volume comprising copper chloride and at least one chloride of the metals selected from the alkali metals, the alkaline earth metals and the rare earth metals supported on gamma alumina hollow cylindrical granules, endowed with high performance in terms of selectivity and conversion combined with specific productivity to DCE (g DCE/g catalyst.h), higher than that of catalysts having the same geometrical parameters (shape and size) and composition.
This and other objects are accomplished by the catalysts of the present invention.
Other objects will be apparent from the following description of the invention.